Open-loop torque control with closed-loop feedback

ABSTRACT

A variator torque control system adjusts a variator output so that the actual output torque of the variator closely matches an expected output torque. In an example, pressure values of an existing torque control map are supplemented in real time with calculated pressure supplement values based on the current operation of the variator. The pressure supplement value for each mapped pressure value may be derived based on a prior application of the same or another map value.

TECHNICAL FIELD

This patent disclosure relates generally to hydrostatic torquecontrolling transmissions, and, more particularly to a system forcompensating for inaccuracies in a variator torque control map.

BACKGROUND

Many sophisticated transmission systems such as continuously variabletransmissions (CVTs) employ a torque controlling element to provide acontinuously variable torque or speed transmission capability. Anexample of such a transmission is a split torque transmission, wherein adrive train is powered by dual inputs, one of which may be atorque-controlled input, such as from a hydraulic variator. In suchsystems, it is generally desirable to be able to accurately control thevariator such that the resultant actual operation of the system based oncontrol signals corresponds to the expected operation.

In attempting to attain this goal, some systems utilize a calibrationmap or torque control map that maps an input pressure or pressuredifferential to an output torque of the variator. Nonetheless, underactual operating conditions, some entries in the torque control map maybe erroneous due to the wear of components, play or slop in the controlsystem, and so on, leading to an undesired discrepancy between theexpected and actual operation of the system.

The foregoing background discussion is intended solely to aid thereader. It is not intended to limit the invention, and thus should notbe taken to indicate that any particular element of a prior system isunsuitable for use within the invention, nor is it intended to indicateany element, including solving the motivating problem, to be essentialin implementing the innovations described herein. The implementationsand application of the innovations described herein are defined by theappended claims.

BRIEF SUMMARY OF THE DISCLOSURE

In one aspect, a method is provided for controlling an output torque ofa variator having a hydraulic actuator responsive to an actuatorpressure signal. The method of this aspect includes receiving anindication of a first desired torque from an operator interface. Aplurality of parameters related to operation of the variator areevaluated and mapped to a first mapped value for the actuator pressuresignal. The first mapped value is applied to the hydraulic actuator asan actuator pressure signal and a first actual output torque of thevariator is measured and compared to the first actual output torque toderive a pressure supplement value. When an indication of a seconddesired torque is received, the plurality of parameters related tooperation of the variator are reevaluated to arrive at a second mappedvalue for the actuator pressure signal which is then modified via thepressure supplement value to produce an adjusted actuator pressuresignal.

Additional and alternative features and aspects of the disclosed systemand method will be appreciated from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a detailed schematic drawing of a variator for providing avariable output torque based on an applied control pressuredifferential;

FIG. 2 is a detailed schematic drawing of a hydraulic actuator forcontrolling the position of a variable-angle swash plate in a variatorsuch as that shown in FIG. 1;

FIG. 3 is a three-dimensional section of a four dimensional mapcorrelating actuator pressure differentials, variator input speeds andvariator output speeds with expected output torques at a variator inputspeed of 1800 RPM;

FIG. 4 is a simplified logical schematic of control components and dataflow associated with an actuator to effectively control a variator;

FIG. 5 is a flowchart illustrating a process for supplementing thevalues of a torque control map according to one example in order toimprove the correspondence between actual and expected torques; and

FIG. 6 is a flowchart illustrating a further process for supplementingthe values of a torque control map in order to improve thecorrespondence between actual and expected torques.

DETAILED DESCRIPTION

This disclosure relates to a system and method for improved variatortorque control system. Using the described system, a variator output istorque-controlled so that the actual output torque of the variator moreclosely matches the desired output torque. Torque control mappings canbe prone to inaccuracies due to operating environment variations,machine variations, tolerance changes, and so on. In one example, thedescribed torque control system adds a calculated pressure supplementvalue to map values before each application thereof to improve thecorrelation between the desired and actual torque of the variator.Additional and alternative aspects will become apparent fromconsideration of the following.

FIG. 1 is a detailed schematic drawing of a variator 100 for providing avariable output torque based on an applied control pressure differentialin the swash plate actuator 104. The variator 100 comprises a pump 101and a motor 102. The pump 101 comprises a variable angle swash plate 103set by a swash plate actuator 104. A number of pistons 105 in respectivechambers ride on the swash plate 103 via sliding contacts, such that therange of movement of the pistons 105 is set by the angle of the swashplate 103. The chambers for the pistons 105 are formed in a pump carrier108 that is rotated via the pump input shaft 109.

The motor 102 comprises a similar arrangement including a number ofpistons 106 in respective chambers. The pistons 106 of the motor 102 areslidably engaged upon a fixed swash plate 107. It will be appreciatedthat the angle of swash plate 107 may also be variable, so as to allow avariable displacement. The chambers of the pistons 105 of the pump 101are in fluid communication with the chambers of the pistons 106 of themotor 102 via hydraulic fluid that fills the chambers and interveningconduits (not shown). The chambers for the pistons 106 are formed in amotor carrier 110 that rotates the motor output shaft 111. As the angleof the swash plate 103 is varied, the amount of fluid displaced by thepistons 105 of the pump 101 (and thus the fluid volume received or takenfrom the chambers of the pistons 106) varies.

Because of these interrelationships, the torque varies with the netforce applied to the swash plate 103 and the output speed of the motor102 varies with the angle of swash plate 103. In overview, the swashplate actuator 104, which in this example operates on differentialhydraulic pressure, is driven via solenoid valves (not shown), e.g., onefor each of two pressure values, controlled electronically byappropriate input signals from a transmission controller or the like. Inthis way, the controller can control the torque of the variator 100 viathe application of electrical signals to solenoid valves associated withthe swash plate actuator 104.

FIG. 2 is a detailed schematic drawing of a hydraulic actuator 104 forcontrolling the actuation force on the variable-angle swash plate (notshown) in a variator 100 such as that shown in FIG. 1. The actuator 104includes a number of interrelated elements including primarily twoopposed pistons 200, 201, within respective cylinders 202, 203. Thepistons 200, 201 cooperate with the bores of their respective cylinders202, 203 to form respective pressure chambers 204, 205 for containingpressurized hydraulic fluid.

The pistons 200, 201 are joined by a bar 206 which has a central pivotpin 207 mounted thereon. The central pivot pin 207 interferes within aslot 208 in a swash plate arm 209, such that the lateral position of thebar 206 establishes the position of the swash plate arm 209 and hencethe angle of the swash plate itself (not shown). The bar 206 is biasedto a central position by opposing springs 212. As the bar 206 isdisplaced from this central position, there is a restoring force exertedby springs 212 that is proportional to the displacement.

The lateral position, velocity, and acceleration of the bar 206 isdetermined by the sum of the forces acting on the pistons 200, 201. Theforces acting on the pistons 200, 201 are derived from the followingsources: (1) pressures in chambers 204 and 205, (2) forces from thespring 212, which are a function of displacement of pistons 200, 201,and (3) swivel forces acting through the swash plate which are afunction of torque, pump speed, motor speed, etc. Respective pressurevalves 210, 211 independently control the pressure within chambers 204,205. In an example, the pressure valves 210, 211 are solenoid valvesthat supply hydraulic fluid at a pressure that is set by an appliedcurrent within limits set by a supply pressure. Thus, in the illustratedexample, each valve 210, 211 has at least a current input (illustratedas inputs A and C) and a fluid input (illustrated as inputs B and D).Typically, solenoid valves can supply fluid at any pressure between zeroand the fluid pressure at the fluid input B, D.

Considering FIG. 2 in conjunction with FIG. 1, it will be appreciatedthat the torque supplied at output 111 is directly related to thepressure differential applied by valves 210, 211. In particular, thefluid pressure within the hydraulic circuit is related to the pressuredifferential applied by valves 210, 211. Thus, in torque-controlledapplications, it is desirable to accurately correlate combinations ofsolenoid currents for valves 210 and 211 (or applied pressuredifferential in actuator 104) with expected associated output torques atoutput 111.

As a first step, a predetermined map is used to correlate specificpressure differentials with specific expected output torques. Inpractice, the relationship between these values also depends upon thefollowing: (1) pump displacement of piston 104 (measured directly via adisplacement sensor, or calculated by motor speed/pump speed, e.g., vianormalized motor speed), and (2) input (pump) speed. Thus, a4-dimensional mapping is used to correlate the various values.

FIG. 3 illustrates such a map with the un-shown dimension of variatorinput speed set at 1800 RPM. Thus, the illustrated surface 300correlates expected output torque (left horizontal axis) with acombination of the applied pressure differential in the actuator 104(vertical axis) and the known variator normalized motor speed ordisplacement (right horizontal axis, normalized). Different absolutevariator input speeds would result in different 3-dimensional surfacesrelating the remaining variables.

In an embodiment, a specialized feedback loop provides a real timeadjustment to the values provided by the map so that the actual torqueoutput of the variator 100 more closely matches the desired torque.Before discussing the variator control process in detail, the controlinfrastructure and informational flow within the system will bediscussed. FIG. 4 is a simplified logical schematic 400 of the controlcomponents and data flow associated with the mechanical components ofFIG. 2 to effectively control the variator 100. In particular, avariator controller 401 is provided for controlling the operation of thevariator 100 via solenoid valves 210 and 211. The variator controller401 may be a dedicated variator controller, but more typically will alsocontrol a larger system, such as a transmission, associated with thevariator 100. The controller 401 may be of any suitable construction,however in one example it comprises a digital processor system includinga microprocessor circuit having data inputs and control outputs,operating in accordance with computer-readable instructions stored on acomputer-readable medium. Typically, the processor will have associatedtherewith long-term (non-volatile) memory for storing the programinstructions, as well as short-term (volatile) memory for storingoperands and results during (or resulting from) processing.

In operation, the controller 401 receives a number of data inputs fromthe variator system 100 and provides a number of control outputs to thesystem 100. In particular, the controller 401 has a first data inputconnected to circuit pressure sensors 402 or other torque sensingdevices or sensors. Although it is possible to use a single pressuresensor, it is desirable to use multiple sensors to obtain more accuratepressure readings. The circuit pressure sensors 402 are positioned andadapted to sense the hydraulic pressure within the internal hydrauliccircuit of the variator 100 (i.e., between pistons 105 and 106) and toprovide signals related to the sensed pressures. A second data input tothe controller 401 is linked to a pump speed sensor 403. The pump speedsensor 403 is positioned and adapted to detect the rotational speed ofthe variator input shaft 108 and to provide a signal related to thesensed rotational input speed. A motor speed sensor 404 is linked tothird data input of the controller 401. The motor speed sensor 404 ispositioned and adapted to detect the rotational speed of the variatoroutput shaft 110 and to provide a signal related to the sensedrotational output speed. It will be appreciated that the pumpdisplacement (e.g., derived from the stroke of actuator 104) or theangle of the swash plate 103 (e.g., derived from an angle sensor) can beused as an input in place of the normalized motor speed.

In order to detect a desired torque, the controller 401 also receives adata input from the operator interface 407, e.g., an acceleratorsetting. The operator may be human or automated, and the operatorinterface may vary accordingly. The variator controller 401 also reads a4-D output map 300 such as that shown in FIG. 3.

Based on the various available inputs as discussed above, the controller401 calculates and provides appropriate control signals such that thevariator 100 provides an output torque closely corresponding to thedesired output torque. In particular, the controller 401 provides twoadjusted solenoid control signals 405, 406 to control the operation ofthe actuator 104 and thus the operation of the variator 100. Theadjusted solenoid control signals 405, 406 include a first adjustedsolenoid control signal 405 to control a first one 210 of the actuatorpressure valves and a second adjusted solenoid control signal 406 tocontrol a second one 211 of the actuator pressure valves.

FIG. 5 is a flowchart 500 illustrating a process for supplementing thevalues of the map 300 according to one example in order to improve thecorrespondence between actual and expected torques. In a first stage501, the controller 401 calculates a desired torque from informationreceived at the operator interface 407. The desired torque may be avalue that is directly calculated from operator input, e.g., acceleratorposition, or that is indirectly calculated, e.g., from transmissionoperation, wherein the state of the transmission is based on current andpast operator inputs. At stage 502, the controller 401 reads thevariator state including the circuit pressure from circuit pressuresensors 402, the pump speed from pump speed sensor 403, and the motorspeed from the motor speed sensor 404.

The controller 401 reads the map 300 at stage 503 and identifies aneeded actuator pressure differential to yield the desired torque. Atstage 504, the controller 401 adds a pressure supplement value P₊ to theidentified actuator pressure differential to produce an adjustedactuator pressure differential. If the process 500 is being executed ona first pass, the pressure supplement value P₊ may be set at zero or aninitial default offset. If the process 500 is being executed on a secondor subsequent pass, the pressure supplement value P₊ will have been seton the prior pass in the manner discussed below.

At stage 505, the controller 401 outputs adjusted solenoid currentsignal 1 (405) and adjusted solenoid current signal 2 (406) based on theadjusted actuator pressure differential of stage 504. At stage 506, thecontroller 401 again reads the circuit pressure from circuit pressuresensor 402 and calculates the actual output torque of the variator 100.In particular, those of skill in the art will appreciate that the outputtorque of a variator is related to and can be directly calculated fromthe internal hydraulic pressure of the variator.

In stage 507, the controller 401 compares the desired torque from stage501 with the actual torque from stage 506 and produces an error pressuredifferential signal P_(e) representing the difference between thedesired pressure (based on the desired torque) and the actual pressure(based on the actual torque). At stage 508, the controller 401 applies again G to the error pressure differential signal P_(e) to produce apressure supplement value P₊. In an example, the gain is amultiplicative gain such that G×P_(e)=P₊. However, the nature andapplication of gain within the scope of this disclosure is not limitedby the foregoing example. Thus, for example, it will be appreciated thatthe gain may be proportional, integral, and/or derivative (PID).Moreover, the gain may be variable or static, and in one example thegain G is a unitless fraction, e.g., 0.5. From stage 508, the processreturns to stage 501 to again read the desired torque.

By executing the process 500, the impact of any inaccuracies in the map300 are minimized so that the actual output torque of the variator 100more closely matches the desired torque expressed at the operatorinterface. It will be appreciated that unless the gain G is set to 1 andthe conditions of the variator remain largely static between passes ofthe process 500, the actual torque will typically not precisely matchthe desired torque, but the difference between actual and desired torquewill generally be substantially lessened over that produced by use ofthe map 300 without correction. The ability to closely match the desiredand actual torque is valuable in many contexts. For example, in manytransmissions, input torque is controlled during shifting to ensuresmooth shifts. In such contexts, the ability to produce an actual outputtorque that closely correlates to an expected output torque will thusenhance the quality of shifts.

In an alternative embodiment, the torque control process operates viatorque command correction rather than pressure correction. Thisembodiment is particularly well-suited for systems wherein thecorrelation between torque control maps is nonlinear in one or morevariables. Stages 601-606 of the torque control process 600 are similarto stages 501-506 of process 500. In stage 607, the controller 401compares the desired torque from stage 601 with the actual torque fromstage 606 and produces a torque error signal T_(e). At stage 608, thecontroller 401 adds T_(e) to a present desired torque to generate acorrected desired torque. It will be appreciated that if the desiredtoque from the operator interface 407 changes between the execution ofstage 601 and the execution of stage 608, the new desired torque is usedin stage 608. From stage 608, the process returns to stage 602 tocalculate the actuator pressure differential necessary to yield thecorrected desired torque. It will be appreciated that on second andsubsequent consecutive passes through process 600, the corrected desiredtorque is used in lieu of the desired torque.

INDUSTRIAL APPLICABILITY

The industrial applicability of the variator torque control systemdescribed herein will be readily appreciated from the foregoingdiscussion. A technique is described wherein a variator output istorque-controlled so that the actual output torque of the variatorclosely matches the desired output torque. Torque control mappings arepredetermined and become inaccurate due to operating environmentvariations, machine variations, tolerance changes, and so on. Thedescribed torque control system adds a calculated pressure supplementvalue to each map value before each application of that map value toimprove the correlation between the desired or expected and actualoutput torque of the variator. In an example, the pressure supplementvalue is derived based on the immediately prior application of the sameor another map value.

Examples of the present disclosure are applicable to any systememploying a hydraulic variator wherein it is desired that the outputtorque of the variator closely match an expected output torque value.For example, many transmission systems, especially for heavy industrialmachines, use components such as constant velocity transmissions thatemploy a variator and that thus may benefit from application of theteachings herein. In such machines, application of the foregoingteachings can provide better shift performance and an improved userexperience due to more precise torque control at the transmission input(i.e., the variator output). Thus, for example, a heavy industrialmachine employing such a transmission may be operated for long timeperiods and in widely varied operating environments without experiencingvariator torque-related errors in shift behavior. Thus, although avariator torque-control map may become inaccurate over time and/oracross different environments, the shift quality of the associatedtransmission may nonetheless be maintained through use of the presentsystem.

It will be appreciated that the foregoing description provides examplesof the disclosed system and technique. However, it is contemplated thatother implementations of the disclosure may differ in detail from theforegoing examples. All references to the invention or examples thereofare intended to reference the particular example being discussed at thatpoint and are not intended to imply any limitation as to the scope ofthe invention more generally. All language of distinction anddisparagement with respect to certain features is intended to indicate alack of preference for those features, but not to exclude such from thescope of the invention entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context.

Accordingly, this invention includes all modifications and equivalentsof the subject matter recited in the claims appended hereto as permittedby applicable law. Moreover, any combination of the above-describedelements in all possible variations thereof is encompassed by theinvention unless otherwise indicated herein or otherwise clearlycontradicted by context.

1. A method of controlling an output torque of a variator having ahydraulic actuator responsive to an actuator pressure signal, the methodcomprising: receiving an indication of a first desired torque from anoperator interface; evaluating a plurality of parameters related tooperation of the variator; reading a map to link the plurality ofparameters to a first mapped value for the actuator pressure signal,wherein the first desired torque is associated with the first mappedvalue; applying the first mapped value to the hydraulic actuator as anactuator pressure signal; measuring a first actual output torque of thevariator and comparing the first actual output torque to the firstdesired output torque to derive an error value; deriving a supplementvalue comprising one of a pressure supplement value and a motor torquesupplement value based on the error value, wherein deriving a pressuresupplement value based on the error value includes multiplying the errorvalue by a gain factor to yield a pressure supplement value; receivingan indication of a second desired torque from the operator interface;reevaluating the plurality of parameters related to operation of thevariator; reading the map to link the plurality of variables to a secondmapped value for the actuator pressure signal, wherein the seconddesired torque is associated with the second mapped value; modifying thesecond mapped value via the pressure supplement value to produce anadjusted actuator pressure signal; and applying the adjusted actuatorpressure signal to the hydraulic actuator to control the output torqueof the variator.
 2. The method according to claim 1, wherein receivingan indication of a first desired torque from the operator interfaceincludes receiving a signal from an accelerator interface.
 3. The methodaccording to claim 1, wherein the actuator pressure signal correspondsto a pressure differential.
 4. The method according to claim 3, whereinthe actuator pressure signal includes at least two solenoid currentsignals directed to at least two respective solenoid valves forcontrolling the actuator.
 5. The method according to claim 1, whereinmodifying the second mapped value via the pressure supplement valueincludes adding the pressure supplement value to the second mapped valueto produce the adjusted actuator pressure signal.
 6. The methodaccording to claim 1, wherein the gain factor is selected from the groupconsisting of a variable gain value and a static gain value.
 7. Themethod according to claim 1, wherein the variator includes an internalhydraulic circuit, a pump, and a motor, and wherein the plurality ofparameters related to operation of the variator include a circuitpressure of the internal hydraulic circuit, a variator pump speed, and avariator motor speed.
 8. The method according to claim 7, whereinmeasuring a first actual output torque of the variator includesconverting the circuit pressure to the first actual output torque.